Scott Turner: When disease appears, so do insects and cleanup crews

Despite their garish colors, the two adult pigeon horntails blended into the gray, ridged bark of the American elm tree, fading fast from Dutch elm disease.Cylindrical, circled in black, red and yellow,...

Comment

By
Scott Turner
Posted Jul. 20, 2013 @ 12:01 am

Despite their garish colors, the two adult pigeon horntails blended into the gray, ridged bark of the American elm tree, fading fast from Dutch elm disease.

Cylindrical, circled in black, red and yellow, and up to two inches in length, the pigeon horntail is a non-stinging wasp that seeks out weakened or dying shade trees to lay its eggs

The insectís name comes from the horn- and spear-like, egg-laying structure attached to the end of the abdomen. That long point is called an ovipositor. A female adult pigeon horntail inserts her ovipositor as much as three quarters of an inch into the wood of a tree trunk or branch to deposit eggs. She delivers the eggs individually, but each female may lay up to 400 eggs.

In early July, a colleague discovered the two horntails on a dying elm near the famed Van Wickle Gates on the Front Green of the Brown University campus. We watched the insects crawl around the trunk, and then one of the horntails began to lay eggs methodically via its ovipositor.

Earlier in the growing season, Brown had treated the 10-to-15-year-old infected elm by pruning off dying branches, and injecting a preventative fungicide into the tree, but it continued to decline. Dutch elm disease spreads through a highly susceptible tree in a matter of weeks.

It takes about a month for horntail eggs to hatch, and then the tubular, cream-colored larvae begin to chew their way into a tree. Some of those larvae grow up to two inches in length.

Pigeon horntail larvae spend two to three years in a tree, chewing tunnels that stretch up to two feet. A sawdust-like material fills each tunnel. The larvae pupate in the tunnels in parchment-like cocoons. The adults emerge in late summer and early fall.

Every season is a chapter in the book of life, with plots and sub-plots. Dutch elm disease, for example, reveals itself in spring and summer, because the ailment attacks the vessels within a tree that carry water and dissolved minerals from roots to canopy. One day a tree is green, and the next it begins to falter.

The pigeon horntails found the right tree. But they picked the wrong time and place.

Sanitation is crucial to containing Dutch elm disease. About 10 days after the horntails laid their eggs and left, Brown removed the tree, grinding the stump into sawdust to help prevent spread of the fungus to surrounding healthy elms. Typically, infected elm wood is chipped, or otherwise destroyed.

During the fall planting season, Brown will plant a new elm nearby. Over the years, when previous elms needed replacing, grounds personnel planted resistant versions of American elm, including the cultivars, Liberty, Valley Forge and New Harmony. But no cultivar is completely immune to the disease.

Brownís 82 American elms are considered one of the nationís largest and oldest native elm collections on a college or university campus.

American elms mature into centuries-old, vase-shaped, multi-stemmed giants. At Brown, the relatively high canopies of such established trees allows for unobstructed sight lines between people and campus architecture.

The pigeon horntail is considered a strong flier. It seldom shows up in large enough numbers to cause significant damage to a tree. The insect also lays eggs on weakened or dying ash, beech, cottonwood, hickory, maple, sycamore and other types of trees.

Given its color, shape, size and behavior, the pigeon horntail is a spectacular creature to observe. Catching horntails in action ranks as the seasonís singular wildlife sighting.